A sensitive method for visualization of brain receptors by in vitro autoradiography featuring intact cell morphology was developed previously in this laboratory. By this method we have mapped the localization of opiate receptors in the brains of rats and other mammalian and non-mammalian vertebrates. Comparisons of tritiated naloxone binding with tritiated enkephalin binding have reinforced the notion of opiate receptor subtypes. These have been followed ontogenetically and phylogenetically and have been related to analogous receptor subtypes in the dopamine system. Applications are being pursued for the study of receptors and biologically active peptides and enzymes in unfixed brains of humans obtained at autopsy. The unfixed, cryostat-cut tissue is amenable for concurrent study of metabolic and functional mapping by 2-deoxy-D-glucose. One project underway will localize and compare phencyclidine (PCP) receptors in the brain with the altered brain metabolic activity produced by phencyclidine "anesthesia". The technqiue is used also for studies related to sleep, circadian rhythms and drug-induced sedation.